1. Field of the Invention
The present invention relates, in general, to a riser system of the type that is typically employed with offshore facilities for transferring fluids, such as oil or gas, from sub sea pipes to a floating tanker vessel that is secured to a single point mooring. The riser system includes a tensioning mechanism that enables the riser to move an amount that is sufficient to accommodate movement of the vessel under extreme conditions or in relatively shallow water, while automatically taking up excess slack that could otherwise damage the riser system.
2. Description of the Related Art
Seagoing vessels often cannot be accommodated in conventional harbors due to the size of the vessel, the capacity of the harbor or for other reasons. In those instances where a conventional mooring is impossible or unavailable, offshore mooring systems must be employed. In situations involving tanker vessels for transporting fluid cargo, systems have been developed wherein a vessel is moored at sea in relatively deep water using what is known as a single point mooring. Associated with the mooring is a riser system that facilitates transfer of the fluid cargo between the vessel and onshore facilities through underwater pipelines. The specific fluid carrying lines that ascend from the sea floor to a vessel are known as risers.
In order to provide sufficient flexibility to allow for the movement of the vessel or single point mooring in response to wave and wind action, the risers are typically made of flexible unbonded steel pipes, hoses or combinations of both, and are usually disposed in catenary type arrangements. Such systems require a minimum length of the riser in order to provide sufficient scope to cover the full range of movements of the vessel. In many applications, this is not possible due to the need to keep the riser clear of the seabed and vessel. In particular, if the riser is made long enough to accommodate a large degree of movement of the vessel, as may occur during extreme weather conditions, then during normal conditions, the extra length of riser will create so much slack therein that the riser will likely contact either the seabed or the vessel, thus potentially damaging the riser. Similarly, in shallow water conditions, even a normal amount of slack in the riser can cause potentially damaging contact of the riser with the seabed.
Therefore, there has been a long felt need for a riser system that is capable of providing a sufficient degree of movement to accommodate vessel movement in extreme conditions or shallow water conditions, but at the same time prevents the generation of excessive slack in the riser that could cause damage thereto.